I Have Had Abnormal TSH Results, 3 Weeks Ago They

I have had abnormal TSH results, 3 weeks ago they were at 12.8 and two weeks ago at 9.8. The doctor prescribed levothyroxine 100mcg. Before I start the treatment I would like to understand what are the implications. Once we introduce the hormone replacement does the thyroid produce even less of its natural hormone ? Wouldn't it create a feedback with the information of not producing more of the natural hormones because there is already plenty? Leading to a cycle with a long life treatment?
I also have some shorten o breath could that be related to the problem with the thyroid or my anxiety?